Method for using terminal to detect small-scale cell in environment in which macrocell and small-scale cell coexist

ABSTRACT

The present disclosure provides a method for using a terminal to detect a small-scale cell at the edge of coverage of the small-scale cell in a wireless communication system in which a macrocell and the small-scale cell coexist. The present invention may include: a step of measuring a macrocell; a step of comparing a value related to the reference signal received quality (RSRQ) acquired according to the macrocell measurement with at least one critical value; a step of determining whether or not to carry out at least one interference removal function for receiving a signal from a small-scale cell in accordance with the determined results; and a step of detecting the signal from the small-scale cell when the interference removal function is actuated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication systems in whicha macro cell and a small-scale cell coexist.

2. Related Art

3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) thatis an advancement of UMTS (Universal Mobile Telecommunication System) isbeing introduced with 3GPP release 8. In 3GPP LTE, OFDMA (orthogonalfrequency division multiple access) is used for downlink, and SC-FDMA(single carrier-frequency division multiple access) is used for uplink.To understand OFDMA, OFDM should be known. OFDM may attenuateinter-symbol interference with low complexity and is in use. OFDMconverts data serially input into N parallel data pieces and carries thedata pieces over N orthogonal sub-carriers. The sub-carriers maintainorthogonality in view of frequency. Meanwhile, OFDMA refers to amultiple access scheme that realizes multiple access by independentlyproviding each user with some of sub-carriers available in the systemthat adopts OFDM as its modulation scheme.

Recently, 3GPP LTE-Advanced (LTE-A) which is an evolution of 3GPP LTEhas been discussed.

In addition, a hetero-network in which a macro cell and a small-scalecell coexist has been discussed recently. Particularly, discussions havebeen progressed in order to detour traffic by dispersing terminalsconnected to a macro cell into a small-scale cell.

However, coverage of the small-scale cell is anticipated to be verynarrow and it is highly probable that a plurality of terminals islocated outside of the coverage of small-scale cell. Accordingly, theeffort to disperse the traffic may be useless.

SUMMARY OF THE INVENTION

The present specification introduces methods which enable a terminallocated outside of coverage of a small-scale cell to access thesmall-scale cell in an environment in which a macro cell and asmall-scale cell coexist.

To achieve the object, according to tone embodiment of the presentspecification, there is provided a method for detecting a small-scalecell in a wireless communication system in which a macro cell and thesmall-scale cell coexist. The method may performed by a user equipmentwhich is located outside a coverage of the small-scale cell. And themethod may comprise: performing a measurement for the macro cell;comparing a value related to a reference signal received quality (RSRQ)obtained by the measurement for the macro cell with at least onethreshold value; determining whether to operate at least oneinterference removing function in order to receive a signal from thesmall-scale cell, according to a result of comparing the value; anddetecting the signal from the small-scale cell, if the interferenceremoving function is operated.

The comparison step may include determining whether the value related tothe RSRQ is between a lower limit value and an upper limit value.

The determining step may include: determining whether to operate atleast one of an interference removing function for a primarysynchronization signal (PSS) and a secondary synchronization signal(SSS), and interference removing function for a physical broadcastchannel (PBCH).

The method may further comprise: performing a cell reselection or ahandover, if a signal is detected from the small-scale cell.

To achieve the object, according to tone embodiment of the presentspecification, there is provided a method for controlling a userequipment in a wireless communication system in which the macro cell anda small-scale cell coexist. The method may performed by a macro cell andcomprise: receiving, by the macro cell, a result of a reference signalreceived quality (RSRQ) measurement for the macro cell itself from auser equipment which is located outside a coverage of the small-scalecell; comparing a value related to the RSRQ with at least one thresholdvalue; and transmitting a control signal for allowing the user equipmentto perform a cell reselection or a handover to the small-scale cellaccording to a result of comparing the value.

The control signal for allowing the user equipment to perform a cellreselection or a handover to the small-scale cell may be either one of acommand instructing the cell reselection or the handover to thesmall-scale cell or a command for instructing the user equipment tooperate an interference removing function.

The command for instructing the user equipment to operate theinterference removing function may corresponds to a command forinstructing the user equipment to operate at least one of aninterference removing function for a primary synchronization signal(PSS) and a secondary synchronization signal (SSS), and an interferenceremoving function for a physical broadcast channel (PBCH).

ADVANTAGEOUS EFFECTS

According to the disclosure of the present specification, a terminalslocated outside of coverage of a small-scale cell is able to access thesmall-scale cell in an environment in which a macro cell and asmall-scale cell coexist.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless communication system.

FIG. 2 illustrates a general multiple antenna system.

FIG. 3 illustrates the architecture of a radio frame according to FDD in3GPP LTE.

FIG. 4 illustrates an example resource grid for one uplink or downlinkslot in 3GPP LTE.

FIG. 5 illustrates the architecture of a downlink sub-frame.

FIG. 6 illustrates the architecture of an uplink sub-frame in 3GPP LTE.

FIG. 7 illustrates an example of comparison between a single carriersystem and a carrier aggregation system.

FIG. 8 illustrates an example in which three DL CCs (DL CC A, DL CC B,and DL CC C) are aggregated, and DL CC A is set as a PDCCH monitoring DLCC.

FIG. 9 illustrates an example of scheduling performed when cross-carrierscheduling is configured in a cross-carrier scheduling.

FIG. 10 is a block diagram representing a structure of an UE accordingto 3GPP LTE as an example.

FIG. 11 illustrates a frame structure for transmitting a synchronizationsignal in a FDD frame defined in 3GPP LTE.

FIG. 12 illustrates an example of frame structure that transmits asynchronization signal in a TDD frame which is defined in 3GPP LTE.

FIG. 13 illustrates an example of a cell detection and a cell selectionthrough a synchronization signal.

FIG. 14 illustrates an example of multimedia broadcast/multicast service(MBMS).

FIG. 15 illustrates a hetero-network that includes a macro cell and asmall-scale cell.

FIG. 16 illustrates an example of the enhanced inter-cell interferencecoordination (eICIC) to solve the problem of interference between BSs.

FIG. 17 illustrates a concept of expanding coverage of a small-scalecell.

FIG. 18 illustrates the interference between signals of a macro cell andsynchronization signals of a small-scale cell and the interferencebetween reference signals.

FIG. 19 is a flowchart illustrating operation of UE according to anembodiment suggested in the present specification.

FIG. 20 is a flowchart illustrating operation of a BS of a macro cellaccording to an embodiment suggested in the present specification.

FIG. 21 is a block diagram illustrating a wireless communication systemwhere an embodiment of the present invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technical terms used herein are used to merely describe specificembodiments and should not be construed as limiting the presentinvention. Further, the technical terms used herein should be, unlessdefined otherwise, interpreted as having meanings generally understoodby those skilled in the art but not too broadly or too narrowly.Further, the technical terms used herein, which are determined not toexactly represent the spirit of the invention, should be replaced by orunderstood by such technical terms as being able to be exactlyunderstood by those skilled in the art. Further, the general terms usedherein should be interpreted in the context as defined in thedictionary, but not in an excessively narrowed manner.

The expression of the singular number in the specification includes themeaning of the plural number unless the meaning of the singular numberis definitely different from that of the plural number in the context.In the following description, the term ‘include’ or ‘have’ may representthe existence of a feature, a number, a step, an operation, a component,a part or the combination thereof described in the specification, andmay not exclude the existence or addition of another feature, anothernumber, another step, another operation, another component, another partor the combination thereof.

The terms ‘first’ and ‘second’ are used for the purpose of explanationabout various components, and the components are not limited to theterms ‘first’ and ‘second’. The terms ‘first’ and ‘second’ are only usedto distinguish one component from another component. For example, afirst component may be named as a second component without deviatingfrom the scope of the present invention.

It will be understood that when an element or layer is referred to asbeing “connected to” or “coupled to” another element or layer, it can bedirectly connected or coupled to the other element or layer orintervening elements or layers may be present. In contrast, when anelement is referred to as being “directly connected to” or “directlycoupled to” another element or layer, there are no intervening elementsor layers present.

Hereinafter, exemplary embodiments of the present invention will bedescribed in greater detail with reference to the accompanying drawings.In describing the present invention, for ease of understanding, the samereference numerals are used to denote the same components throughout thedrawings, and repetitive description on the same components will beomitted. Detailed description on well-known arts which are determined tomake the gist of the invention unclear will be omitted. The accompanyingdrawings are provided to merely make the spirit of the invention readilyunderstood, but not should be intended to be limiting of the invention.It should be understood that the spirit of the invention may be expandedto its modifications, replacements or equivalents in addition to what isshown in the drawings.

As used herein, ‘wireless device’ may be stationary or mobile, and maybe denoted by other terms such as terminal, MT (mobile terminal), UE(user equipment), ME (mobile equipment), MS (mobile station), UT (userterminal), SS (subscriber station), handheld device, or AT (accessterminal).

As used herein, ‘base station’ generally refers to a fixed station thatcommunicates with a wireless device and may be denoted by other termssuch as eNB (evolved-NodeB), BTS (base transceiver system), or accesspoint.

Hereinafter, applications of the present invention based on 3GPP (3rdgeneration partnership project) LTE (long term evolution) or 3GPP LTE-A(advanced) are described. However, this is merely an example, and thepresent invention may apply to various wireless communication systems.Hereinafter, LTE includes LTE and/or LTE-A.

FIG. 1 shows a wireless communication system.

The wireless communication system 10 includes at least one base station(BS) 11. Respective BSs 11 provide a communication service to particulargeographical areas 15 a, 15 b, and 15 c (which are generally calledcells). Each cell may be divided into a plurality of areas (which arecalled sectors). A user equipment (UE) 12 may be fixed or mobile and maybe referred to by other names such as mobile station (MS), mobile userequipment (MT), user user equipment (UT), subscriber station (SS),wireless device, personal digital assistant (PDA), wireless modem,handheld device. The BS 11 generally refers to a fixed station thatcommunicates with the UE 12 and may be called by other names such asevolved-NodeB (eNB), base transceiver system (BTS), access point (AP),etc.

The terminal generally belongs to one cell and the cell to which theterminal belong is referred to as a serving cell. A base station thatprovides the communication service to the serving cell is referred to asa serving BS. Since the wireless communication system is a cellularsystem, another cell that neighbors to the serving cell is present.Another cell which neighbors to the serving cell is referred to aneighbor cell. A base station that provides the communication service tothe neighbor cell is referred to as a neighbor BS. The serving cell andthe neighbor cell are relatively decided based on the terminal.

Hereinafter, a downlink means communication from the base station 20 tothe terminal 10 and an uplink means communication from the terminal 10to the base station 20. In the downlink, a transmitter may be a part ofthe base station 20 and a receiver may be a part of the terminal 10. Inthe uplink, the transmitter may be a part of the terminal 10 and thereceiver may be a part of the base station 20.

Meanwhile, the wireless communication system may be any one of amultiple-input multiple-output (MIMO) system, a multiple-inputsingle-output (MISO) system, a single-input single-output (SISO) system,and a single-input multiple-output (SIMO) system. The MIMO system uses aplurality of transmit antennas and a plurality of receive antennas. TheMISO system uses a plurality of transmit antennas and one receiveantenna. The SISO system uses one transmit antenna and one receiveantenna. The SIMO system uses one transmit antenna and one receiveantenna. Hereinafter, the transmit antenna means a physical or logicalantenna used to transmit one signal or stream and the receive antennameans a physical or logical antenna used to receive one signal orstream.

FIG. 2 illustrates a general multiple antenna system.

As shown in FIG. 1, when increasing the number of transmission antennato N_(T) and increasing the number of reception antenna to N_(R) at thesame time, a transmission rate can be increased and a frequencyefficiency can be dramatically increased because a theoretical channeltransmission capacity is increased in proportion to the number ofantenna, unlike the case of using multiple antennas only in either oneof transmitter or receiver.

The transmission rate due to the increase of channel transmissioncapacity may be theoretically increased by multiple of a maximumtransmission rate R_(o) in case of using an antenna and a rate increaseR_(i) as shown below. That is, for example, in the MIMO communicationsystem that uses 4 transmission antennas and 4 reception antennas, thetransmission rate may be increased 4 times in comparison with the singleantenna system theoretically.

After the theoretical increase of capacity in such a multiple antennasystem is proved in the middle of 1990′, various technologies to inducethe theoretical increase into actual increase of data transmission ratehas been researched up to now, and a few of the technologies are alreadyapplied to various wireless communication standards such as thirdgeneration mobile communication and next generation wireless LAN, etc.

R _(i)=min(N _(T) ,N _(R))  [Equation 1]

The research trends in relation to the multiple antenna up to now showthat researches have been vigorously progressed in various aspects suchas a research in the aspect of information theory in relation tocommunication capacity calculation of multiple antenna in variouschannel environment and multiple access environment, researches ofwireless channel measurement and modeling process of the multipleantenna system, and a research of space-time signal processing forincreasing transmission reliability and transmission rate, etc.

In a user equipment structure having general MIMO channel environment,reception signals received in each reception antenna can be expressed asfollows.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1\; N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{i\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Herein, the channel between respective transmission and receptionantennas may be distinguished based on transmission and reception index,and the channel passing from a transmission antenna j to a receptionantenna i is represented as h_(ij). In case of using precoding schemelike LTE when transmitting a signal, the transmission signal x can beexpressed by Equation 3.

$\begin{matrix}{x = {\left\lbrack \begin{matrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{matrix} \right\rbrack = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{i\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Herein, w_(ij), a precoding matrix w means a weighting between a i^(th)transmission antenna and j^(th) information. In this time, if thetransmission power of a respective signal to be transmitted is P₁, P₂, .. . , P_(N) _(T) , a transmission information of which transmissionpower has been adjusted may be represented as a diagonal matrix P asfollows.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Meanwhile, a wireless communication system may be divided into afrequency division duplex (FDD) method and a time division duplex (TDD)method. Based on the FDD method, an uplink transmission and a downlinktransmission are progressed in different frequency bands. Based on theTDD method, the uplink transmission and the downlink transmission areperformed in the same frequency band at different times. A channelresponse of a TDD method is actually reciprocal. This means the downlinkchannel response and the uplink channel response are almost same in thecurrent frequency domain. Therefore, there is an advantage in that thedownlink channel response in the wireless communication system based onthe TDD may be obtained from the uplink channel response. In the TDDmethod, as the whole frequency domain is divided into an uplinktransmission and a downlink transmission by time-share, it is notavailable to perform the downlink transmission by a terminal and theuplink transmission by a UE at the same time. In the TDD system in whichan uplink transmission and a downlink transmission are divided by asubframe unit, the uplink transmission and the downlink transmission areperformed in different subframes.

Hereinafter, the LTE system is described in further detail.

FIG. 3 illustrates the architecture of a radio frame according to FDD in3GPP LTE.

Referring to FIG. 3, the radio frame is composed of ten subframes, andone subframe is composed of two slots. The slots in the radio frame aredesignated by slot numbers from 0 to 19. The time at which one subframeis transmitted is referred to as a transmission time interval (TTI). TheTTI may be called as a scheduling unit for data transmission. Forexample, the length of one radio frame may be 10 ms, the length of onesubframe may be 1 ms, and the length of one slot may be 0.5 ms.

The structure of the radio frame is merely an example, and the number ofsubframes included in the radio frame, the number of slots included inthe subframe, etc. may be variously modified.

FIG. 4 illustrates an example resource grid for one uplink or downlinkslot in 3GPP LTE.

Referring to FIG. 4, the uplink slot includes a plurality of OFDM(orthogonal frequency division multiplexing) symbols in the time domainand NUL resource blocks (RBs) in the frequency domain. OFDM symbol is torepresent one symbol period, and depending on system, may also bedenoted SC-FDMA symbol, OFDM symbol, or symbol period. The resourceblock is a unit of resource allocation and includes a plurality ofsub-carriers in the frequency domain. The number of resource blocksincluded in the uplink slot, i.e., NUL, is dependent upon an uplinktransmission bandwidth set in a cell. Each element on the resource gridis denoted resource element.

Here, by way of example, one resource block includes 7×12 resourceelements that consist of seven OFDM symbols in the time domain and 12sub-carriers in the frequency domain. However, the number ofsub-carriers in the resource block and the number of OFDM symbols arenot limited thereto. The number of OFDM symbols in the resource block orthe number of sub-carriers may be changed variously. In other words, thenumber of OFDM symbols may be varied depending on the above-describedlength of CP. In particular, 3GPP LTE defines one slot as having sevenOFDM symbols in the case of CP and six OFDM symbols in the case ofextended CP.

In 3GPP LTE, the resource grid for one uplink slot shown in FIG. 4 mayalso apply to the resource grid for the downlink slot.

FIG. 5 illustrates the architecture of a downlink sub-frame.

For this, 3GPP TS 36.211 V10.4.0 (2011-12) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 10)”, Ch. 4 may be referenced.

The radio frame includes 10 sub-frames indexed 0 to 9. One sub-frameincludes two consecutive slots. Accordingly, the radio frame includes 20slots. The time taken for one sub-frame to be transmitted is denoted TTI(transmission time interval). For example, the length of one sub-framemay be 1 ms, and the length of one slot may be 0.5 ms.

One slot may include a plurality of OFDM (orthogonal frequency divisionmultiplexing) symbols in the time domain. OFDM symbol is merely torepresent one symbol period in the time domain since 3GPP LTE adoptsOFDMA (orthogonal frequency division multiple access) for downlink (DL),and the multiple access scheme or name is not limited thereto. Forexample, the OFDM symbol may be referred to as SC-FDMA (singlecarrier-frequency division multiple access) symbol or symbol period.

Here, one slot includes seven OFDM symbols, by way of example. However,the number of OFDM symbols included in one slot may vary depending onthe length of CP (cyclic prefix). That is, as described above, accordingto 3GPP TS 36.211 V10.4.0, one slot includes seven OFDM symbols in thenormal CP and six OFDM symbols in the extended CP.

Resource block (RB) is a unit for resource allocation and includes aplurality of sub-carriers in one slot. For example, if one slot includesseven OFDM symbols in the time domain and the resource block includes 12sub-carriers in the frequency domain, one resource block may include7×12 resource elements (REs).

The DL (downlink) sub-frame is split into a control region and a dataregion in the time domain. The control region includes up to first threeOFDM symbols in the first slot of the sub-frame. However, the number ofOFDM symbols included in the control region may be changed. A PDCCH(physical downlink control channel) and other control channels areassigned to the control region, and a PDSCH is assigned to the dataregion.

As set forth in 3GPP TS 36.211 V10.4.0, the physical channels in 3GPPLTE may be classified into data channels such as PDSCH (physicaldownlink shared channel) and PUSCH (physical uplink shared channel) andcontrol channels such as PDCCH (physical downlink control channel),PCFICH (physical control format indicator channel), PHICH (physicalhybrid-ARQ indicator channel) and PUCCH (physical uplink controlchannel).

The PCFICH transmitted in the first OFDM symbol of the sub-frame carriesCIF (control format indicator) regarding the number (i.e., size of thecontrol region) of OFDM symbols used for transmission of controlchannels in the sub-frame. The wireless device first receives the CIF onthe PCFICH and then monitors the PDCCH.

Unlike the PDCCH, the PCFICH is transmitted through a fixed PCFICHresource in the sub-frame without using blind decoding.

The PHICH carries an ACK (positive-acknowledgement)/NACK(negative-acknowledgement) signal for a UL HARQ (hybrid automatic repeatrequest). The ACK/NACK signal for UL (uplink) data on the PUSCHtransmitted by the wireless device is sent on the PHICH.

The PBCH (physical broadcast channel) is transmitted in the first fourOFDM symbols in the second slot of the first sub-frame of the radioframe. The PBCH carries system information necessary for the wirelessdevice to communicate with the base station, and the system informationtransmitted through the PBCH is denoted MIB (master information block).In comparison, system information transmitted on the PDSCH indicated bythe PDCCH is denoted SIB (system information block).

The control information transmitted through the PDCCH is denoteddownlink control information (DCI). The DCI may include resourceallocation of PDSCH (this is also referred to as DL (downlink) grant),resource allocation of PUSCH (this is also referred to as UL (uplink)grant), a set of transmission power control commands for individual UEsin some UE group, and/or activation of VoIP (Voice over InternetProtocol).

In 3GPP LTE, blind decoding is used for detecting a PDCCH. The blinddecoding is a scheme of identifying whether a PDCCH is its own controlchannel by demasking a desired identifier to the CRC (cyclic redundancycheck) of a received PDCCH (this is referred to as candidate PDCCH) andchecking a CRC error. The base station determines a PDCCH formataccording to the DCI to be sent to the wireless device, then adds a CRCto the DCI, and masks a unique identifier (this is referred to as RNTI(radio network temporary identifier) to the CRC depending on the owneror purpose of the PDCCH.

According to 3GPP TS 36.211 V10.4.0, the uplink channels include aPUSCH, a PUCCH, an SRS (Sounding Reference Signal), and a PRACH(physical random access channel).

FIG. 6 illustrates the architecture of an uplink sub-frame in 3GPP LTE.

Referring to FIG. 6, the uplink sub-frame may be separated into acontrol region and a data region in the frequency domain. The controlregion is assigned a PUCCH (physical uplink control channel) fortransmission of uplink control information. The data region is assigneda PUSCH (physical uplink shared channel) for transmission of data (insome cases, control information may also be transmitted).

The PUCCH for one terminal is assigned in resource block (RB) pair inthe sub-frame. The resource blocks in the resource block pair take updifferent sub-carriers in each of the first and second slots. Thefrequency occupied by the resource blocks in the resource block pairassigned to the PUCCH is varied with respect to a slot boundary. This isreferred to as the RB pair assigned to the PUCCH having beenfrequency-hopped at the slot boundary. The terminal may obtain afrequency diversity gain by transmitting uplink control informationthrough different sub-carriers over time.

FIG. 7 illustrates an example of comparison between a single carriersystem and a carrier aggregation system.

Referring to FIG. 7( a), a typical FDD wireless communication systemsupports one carrier for uplink and downlink. In this case, the carriermay have various bandwidths, but only one carrier is assigned to theuser equipment.

In other words, in the typical FDD wireless communication system, datatransmission and reception is carried out through one downlink band andone uplink band corresponding thereto. The bit stream and the userequipment transmit and receive control information and/or data scheduledfor each sub-frame. The data is transmitted/received through the dataregion configured in the uplink/downlink sub-frame, and the controlinformation is transmitted/received through the control regionconfigured in the uplink/downlink sub-frame. For this, theuplink/downlink sub-frame carries signals through various physicalchannels. Although the description in connection with FIG. 7 primarilyfocuses on the FDD scheme for ease of description, the foregoing may beapplicable to the TDD scheme by separating the radio frame foruplink/downlink in the time domain.

As shown in FIG. 7( a), data transmission/reception performed throughone downlink band and one uplink band corresponding to the downlink bandis referred to as a single carrier system.

Such single carrier system may correspond to an example of communicationin the LTE system. Such 3GPP LTE system may have an uplink bandwidth anda downlink bandwidth that differ from each other, but supports up to 20MHz.

Meanwhile, a high data transmission rate is demanded. The mostfundamental and stable solution to this is to increase bandwidth.

However, the frequency resources are presently saturated, and varioustechnologies are partially being in use in a wide range of frequencyband. For such reason, as a method for securing a broad bandwidth tosatisfy the demand for higher data transmission rate, each scatteredband may be designed to meet basic requirements for being able tooperate an independent system, and carrier aggregation (CA) whoseconcept is to bundle up multiple bands to a single system has beenintroduced.

That is, the carrier aggregation (CA) system means a system thatconstitutes a broadband by gathering one or more carriers each of whichhas a bandwidth narrower than the targeted broadband when supporting abroadband in the wireless communication system.

Such carrier aggregation (CA) technology is also adopted in theLTE-advanced (hereinafter, ‘LTE-A’). The carrier aggregation (CA) systemmay also be referred to as a multiple-carrier system or bandwidthaggregation system.

In the carrier aggregation (CA) system, a user equipment maysimultaneously transmit or receive one or more carriers depending on itscapabilities. That is, in the carrier aggregation (CA) system, aplurality of component carriers (CCs) may be assigned to a userequipment. As used herein, the term “component carrier” refers to acarrier used in a carrier aggregation system and may be abbreviated to acarrier. Further, the term “component carrier” may mean a frequencyblock for carrier aggregation or a center frequency of a frequency blockin the context and they may be interchangeably used.

FIG. 7( b) may correspond to a communication example in an LTE-A system.

Referring to FIG. 7( b), in case, e.g., three 20 MHz component carriersare assigned to each of uplink and downlink, the user equipment may besupported with a 60 MHz bandwidth. Or, for example, if five CCs areassigned as granularity of the unit of carrier having a 20 MHzbandwidth, up to 100 MHz may be supported. FIG. 7( b) illustrates anexample in which the bandwidth of an uplink component carrier is thesame as the bandwidth of a downlink component carrier for ease ofdescription. However, the bandwidth of each component carrier may bedetermined independently. When aggregating one or more componentcarriers, a targeted component carrier may utilize the bandwidth used inthe existing system for backward compatibility with the existing system.For example, in a 3GPP LTE system, bandwidths of 1.4 MHz, 3 MHz, 5 MHz,10 MHz, 15 MHz and 20 MHz may be supported. Accordingly, the bandwidthof an uplink component carrier may be constituted like 5 MHz(UL CC0)+20MHz(UL CC1)+20 MHz(UL CC2)+20 MHz(UL CC3)+5 MHz(UL CC4), for example.However, without consideration of backward compatibility, a newbandwidth may be defined rather the existing system bandwidth beingused, to constitute a broadband.

FIG. 7( b) illustrates an example in which the number of uplinkcomponent carriers is symmetric with the number of downlink componentcarriers for ease of description. As such, when the number of uplinkcomponent carriers is the same as the number of downlink componentcarriers is denoted symmetric aggregation, and when the number of uplinkcomponent carriers is different from the number of downlink componentcarriers is denoted asymmetric aggregation.

The asymmetric carrier aggregation may occur due to a restriction onavailable frequency bands or may be artificially created by a networkconfiguration. As an example, even when the entire system band comprisesN CCs, the frequency band where a particular user equipment may performreception may be limited to M (<N) CCs. Various parameters for carrieraggregation may be configured cell-specifically, UE group-specifically,or UE-specifically.

Meanwhile, carrier aggregation systems may be classified into contiguouscarrier aggregation systems where each carrier is contiguous withanother and non-contiguous carrier aggregation systems where eachcarrier is spaced apart from another. A guard band may be presentbetween the carriers in the contiguous carrier aggregation system.Hereinafter, simply referring to a multi-carrier system or carrieraggregation system should be understood as including both when componentcarriers are contiguous and when component carriers are non-contiguous.

Meanwhile, the concept of cell as conventionally appreciated is variedby the carrier aggregation technology. In other words, according to thecarrier aggregation technology, the term “cell” may mean a pair of adownlink frequency resource and an uplink frequency resource. Or, thecell may mean a combination of one downlink frequency resource and anoptional uplink frequency resource.

In other words, according to the carrier aggregation technology, one DLCC or a pair of UL CC and DL CC may correspond to one cell. Or, one cellbasically includes one DL CC and optionally includes a UL CC.Accordingly, a user equipment communicating with a bit stream through aplurality of DL CCs may be said to receive services from a plurality ofserving cells. In this case, although downlink is constituted of aplurality of DL CCs, uplink may be used by only one CC. In such case,the user equipment may be said to receive services from a plurality ofserving cells for downlink and to receive a service from only oneserving cell for uplink.

Meanwhile, in order for packet data to be transmitted/received through acell, configuration for a particular cell should be completed. Here, theterm “configuration” means the state where system information necessaryfor data transmission/reception on a corresponding cell is completelyreceived. For example, the configuration may include the overall processof receiving common physical layer parameters necessary for datatransmission/reception, MAC (media access control) layer parameters, orparameters necessary for a particular operation in RRC layer. Theconfiguration-completed cell is in the state where packettransmission/reception is possible simply when information indicatingthat packet data may be transmitted is received.

The configuration-completed cell may be left in activation ordeactivation state. Here, the term “activation” refers to datatransmission or reception being performed or being ready. The UE maymonitor or receive a control channel (PDCCH) or data channel (PDSCH) ofan activated cell in order to identify resources (which may be frequencyor time) assigned thereto.

Transmission or reception with a deactivated cell is impossible, whilemeasurement or transmission/reception of least information is possible.The user equipment may receive system information (SI) necessary forreceiving packets from a deactivated cell. In contrast, the userequipment does not monitor or receive the control channel (PDCCH) anddata channel (PDSCH) of deactivated cells to identify resources (whichmay be frequency or time) assigned thereto.

In accordance with carrier aggregation technology, thus,activation/deactivation of a component carrier may be the same inconcept as activation/deactivation of a serving cell. For example,assuming that serving cell 1 comprises DL CC1, activation of servingcell 1 means activation of DL CC1. Assuming that serving cell 2 isconfigured so that DL CC2 is connected with UL CC2, activation ofserving cell 2 means activation of DL CC2 and UL CC2. In that regard,each component carrier may correspond to a serving cell.

On the other hand, a change in the concept of serving cell asconventionally understood by the carrier aggregation technology leads toprimary cells and secondary cells being separated from each other.

The primary cell refers to a cell operating in a primary frequency andmeans a cell where the user equipment performs an initial connectionestablishment procedure or connection re-establishment procedure with abit stream or a cell designated so during the course of handover.

The secondary cell means a cell operating in a secondary frequency, andis configured once an RRC connection is established and is used toprovide additional radio resources.

The PCC (primary component carrier) means a component carrier (CC)corresponding to the primary cell. The PCC means a CC where the userequipment initially achieves connection (or RRC connection) with thebase station among various CCs. The PCC is a special CC that is incharge of connection (or RRC connection) for signaling regardingmultiple CCs and that manages UE context that is connection informationrelating to the UE. Further, the PCC, in case the PCC achievesconnection with the UE so that it is in RRC connected mode, alwaysremains in activated state. The downlink component carrier correspondingto the primary cell is referred to as a downlink primary componentcarrier (DL PCC), and the uplink component carrier corresponding to theprimary cell is referred to as an uplink primary component carrier (ULPCC).

The SCC (secondary component carrier) means a CC corresponding to thesecondary cell. That is, the SCC is a CC assigned to the user equipment,which is not the PCC, and the SCC is an extended carrier for the userequipment to assign additional resources other than the PCC. The SCC maystay in activated state or deactivated state. The downlink componentcarrier corresponding to the secondary cell is referred to as a downlinksecondary component carrier (DL SCC), and the uplink component carriercorresponding to the secondary cell is referred to as an uplinksecondary component carrier (UL SCC).

The primary cell and the secondary cell have the following features.

First, the primary cell is used for transmission of a PUCCH. Second, theprimary cell always remain activated while the secondary cell switchesbetween activation/deactivation depending on particular conditions.Third, when the primary cell experiences radio link failure(hereinafter, “RLF”), the RRC reconnection is triggered. Fourth, theprimary cell may be varied by a handover procedure that comes togetherwith security key changing or an RACH (Random Access CHannel) procedure.Fifth, NAS (non-access stratum) information is received through theprimary cell. Sixth, in the case of an FDD system, the primary cell isconstituted of a pair of DL PCC and UL PCC. Seventh, a differentcomponent carrier may be set as the primary cell for each userequipment. Eighth, primary cells may be exchanged only by a handover,cell selection/cell reselection process. In adding a new secondary cell,RRC signaling may be used to transmit system information of thededicated secondary cell.

As described above, the carrier aggregation system may support aplurality of component carriers (CCs), i.e., a plurality of servingcells, unlike the single carrier system.

Such carrier aggregation system may support cross-carrier scheduling.The cross-carrier scheduling is a scheduling method that allows forresource allocation of a PDSCH transmitted through other componentcarrier through a PDCCH transmitted through a particular componentcarrier and/or resource allocation of a PUSCH transmitted through othercomponent carrier than the component carrier basically linked with theparticular component carrier. That is, a PDCCH and a PDSCH may betransmitted through different downlink CCs, and a PUSCH may betransmitted through an uplink CC other than an uplink CC linked with adownlink CC through which a PDCCH including a UL grant is transmitted.As such, the cross-carrier scheduling-supportive system requires acarrier indicator indicating a DL CC/UL CC through which a PDSCH/PUSCHthrough which a PDCCH provides control information is transmitted. Thefield containing such carrier indicator is hereinafter referred to as acarrier indication field (CIF).

The carrier aggregation system supportive of cross-carrier schedulingmay include a carrier indication field (CIF) in the conventional DCI(downlink control information) format. A cross-carrierscheduling-supportive system, e.g., an LTE-A system, adds a CIF to theexisting DCI format (i.e., DCI format used in LTE), so that it may beextended with three bits, and it may reuse the existing coding scheme,resource allocation scheme (i.e., CCE-based resource mapping) for thePDCCH structure.

FIG. 8 exemplifies cross-carrier scheduling in the carrier aggregationsystem.

Referring to FIG. 8, the base station may configure a PDCCH monitoringDL CC (monitoring CC) set. The PDCCH monitoring DL CC set consists ofsome of all of the aggregated DL CCs, and if cross-carrier scheduling isconfigured, the user equipment performs PDCCH monitoring/decoding onlyon the DL CCs included in the PDCCH monitoring DL CC set. In otherwords, the base station transmits a PDCCH for PDSCH/PUSCH that issubject to scheduling only through the DL CCs included in the PDCCHmonitoring DL CC set. The PDCCH monitoring DL CC set may be configuredUE-specifically, UE group-specifically, or cell-specifically.

FIG. 8 illustrates an example in which three DL CCs (DL CC A, DL CC B,and DL CC C) are aggregated, and DL CC A is set as a PDCCH monitoring DLCC. The user equipment may receive a DL grant for the PDSCH of DL CC A,DL CC B, and DL CC C through the PDCCH of DL CC A. The DCI transmittedthrough the PDCCH of DL CC A contains a CIF so that it may indicatewhich DL CC the DCI is for.

FIG. 9 illustrates an example of scheduling performed when cross-carrierscheduling is configured in a cross-carrier scheduling.

Referring to FIG. 9, DL CC 0, DL CC 2, and DL CC 4 belong to a PDCCHmonitoring DL CC set. The user equipment searches for DL grants/ULgrants for DL CC 0 and UL CC 0 (UL CC linked to DL CC 0 via SIB 2) inthe CSS of DL CC 0. The user equipment searches for DL grants/UL grantsfor DL CC 1 and UL CC 1 in SS 1 of DL CC 0. SS 1 is an example of USS.That is, SS 1 of DL CC 0 is a space for searching for a DL grant/ULgrant performing cross-carrier scheduling.

FIG. 10 is a block diagram representing a structure of an UE accordingto 3GPP LTE as an example.

In the long-term evolution (LTE) or LTE-A, an orthogonal frequencydivision multiplexing (OFDM) is used in downlink, but a single-carrier(SC)-FDMA (similar to OFDM) is used in uplink.

FDMA may be said to be DFT-s OFDM (DFT-spread OFDM). When using theSC-FDMA transmission scheme, the non-linear distortion of poweramplifier may be avoided, thus allowing power consumption-limited userequipment to enjoy increased transmission power efficiency. Accordingly,user throughput may be increased.

SC-FDMA is similar to OFDM in that SC-FDMA also employs FFT (FastFourier Transform) and IFFT (Inverse-FFT). However, the problem with theexisting OFDM transmitters is that signals over each sub-carrier onfrequency axis are converted to signals on time axis by IFFT. That is,IFFT is in the form of performing the same parallel operation, thuscausing an increase in PAPR (Peak to Average Power Ratio). To preventsuch increase in PAPR, SC-FDMA, unlike OFDM, performs IFFT after DFTspreading. In other words, the transmission scheme of performing IFFTafter DFT spreading is referred to as SC-FDMA. Thus, SC-FDMA is alsocalled DFT spread OFDM (DFT-s-OFDM).

Such advantages of SC-FDMA led to being robust for multi-path channelsthanks to similar structure to OFDM while enabling efficient use ofpower amplifier by fundamentally solving the problem of existing OFDMthat OFDM causes increased PAPR due to IFFT operation.

Referring to FIG. 10, a UE 100 includes a RF unit 110. The RF unit 110includes a transmission terminal, that is, a discrete Fourier transform(DFT) unit 111, a subcarrier mapper 112, an IFFT unit 113 and a CPinsertion unit 114, and a radio transmission unit 115. The transmissionterminal of the RF unit 110 further includes, for example, a scrambleunit (not shown), a modulation mapper (not shown), a layer mapper (notshown) and a layer permutator (not shown), and those are arranged aheadof the DFT unit 111. That is, as previously described, in order toprevent an increase of PAPR, the transmission terminal of the RF unit110 has the information gone through the DFT 111 before signals mappedto a subcarrier. The signal that is spread (or precoded in the samemeaning) by the DFT 111 is mapped to a subcarrier through a subcarriermapper 112, and after that, made into a signal on the time axis passingthrough an inverse fast Fourier transform (IFFT) unit again.

That is, due to the correlation among the DFT unit 111, the subcarriermapper 112 and the IFFT unit 113, peak-to-average power ratio (PAPR) oflater time domain signal of the IFFT unit 113 is not significantlyincreased in the SC-FDMA, different from the case of the OFDM, andaccordingly, it is beneficial in the aspect of transmission powerefficiency. That is, in the SC-FDMA, the PAPR or cubic metric (CM) maybe decreased.

The DFT unit 111 outputs complex-valued symbols by performing DFT forthe input symbols. For example, when N_(tx) symbols are inputted (N_(tx)is natural numbers), the size of DFT is N_(tx). The DFT unit 111 may becalled a transform precoder. The subcarrier mapper 112 maps thecomplex-valued symbols to each subcarrier in the frequency domain. Thecomplex-valued symbols may be mapped to the resource elements thatcorrespond to the resource blocks allocated for data transmission. Thesubcarrier mapper 112 may be called a resource element mapper. The IFFTunit 113 outputs baseband signal for data which is a time domain signalby performing IFFT for the inputted symbol. The CP insertion unit 114copies a part of a rear part of the baseband signal for data and insertsit into a front part of the baseband signal for data. The inter-symbolinterference (ISI) and the inter-carrier interference (ICI) areprevented by inserting the CP, thereby orthogonality can be maintainedeven in multi-path channel.

Meanwhile, 3GPP is actively standardizing LTE-Advanced that is anadvanced version of LTE and has adopted clustered DFT-s-OFDM scheme thatpermits non-contiguous resource allocation.

Clustered DFT-s OFDM transmission scheme is a modification of theconventional SC-FDMA transmission scheme, and is a method of mapping bydividing the data symbols that have passed through the precoder into aplurality of subblocks and separating them in the frequency domain. Somemajor features of the clustered DFT-s-OFDM scheme include enablingfrequency-selective resource allocation so that the scheme may flexiblydeal with a frequency selective fading environment.

In this case, the clustered DFT-s-OFDM scheme adopted as an uplinkaccess scheme for LTE-advanced, unlike the conventional LTE uplinkaccess scheme, i.e., SC-FDMA, permits non-contiguous resourceallocation, so that uplink data transmitted may be split into severalunits of cluster.

In other words, while the LTE system is rendered to maintain singlecarrier characteristics in the case of uplink, the LTE-A system allowsfor non-contiguous allocation of DFT_precoded data on frequency axis orsimultaneous transmission of PUSCH and PUCCH. In such case, the singlecarrier features are difficult to maintain.

On the other hand, the RF unit 110 may include a reception terminal, forexample, a radio reception unit 116, a CP removing unit 117, a FFT unit118 and an interference removing unit 119, etc. The radio reception unit116, the CP removing unit 117 and the FFT unit 118 of the receptionterminal perform reverse functions of the radio transmission unit 115the CP insertion unit 114 and the IFFT unit 113.

The interference removing unit 119 removes or alleviates theinterference included in the signal received.

FIG. 11 illustrates a frame structure for transmitting a synchronizationsignal in a FDD frame defined in 3GPP LTE.

A slot number or a subframe number starts from zero. A UE maysynchronize the time and frequency based on a synchronization signalreceived from a BS. The synchronization signal of 3GPP LTE-A is used forperforming a cell search, and may be divided into a primarysynchronization signal (PSS) and a secondary synchronization signal(SSS). For the synchronization signal of 3GPP LTE-A, section 6.11 of3GPP TS V10.2.0 (2011-06) can be referred.

The PSS is used for acquiring OFDM symbol synchronization or slotsynchronization, and is in relation to a physical-layer cell identity(PCI). And the SSS is used for acquiring frame synchronization. Also,the SSS is used for detecting a CP length and acquiring a physical-layercell group ID.

The synchronization signal may be transmitted in subframe 0 and subframe5 respectively in consideration of 4.6 ms, which is global system formobile communication (GSM) frame length, in order to easily performinter-RAT measurement, and the frame boundary may be detected throughthe SSS. In more detail, in the FDD system, the PSS is transmitted inthe last OFDM symbol of 0^(th) slot and 10^(th) slot, and the SSS istransmitted in the OFDM symbol right ahead of the PSS.

The synchronization signal may transmit one of total 504 physical cellID through the combination of 3 PSS and 168 SSS. A physical broadcastchannel (PBCH) is transmitted in first 4 OFDM symbols of a first slot.The synchronization signal and the PBCH are transmitted within 6 RB inthe middle of system bandwidth, and therefore, a UE may detect or decoderegardless of the transmission bandwidth. The physical channel in whichthe PSS is transmitted is referred as P-SCH, and the physical channel inwhich the SSS is transmitted is referred to as S-SCH.

The transmission diversity scheme of synchronization signal uses onlysingle antenna port, and is not defined separately in a standard. Thatis, a single antenna transmission or a transmission scheme transparentto UE (for example, precoding vector switching (PVS), time switchedtransmit diversity (TSTD), and cyclic delay diversity (CDD) may be used.

FIG. 12 illustrates an example of frame structure that transmits asynchronization signal in a TDD frame which is defined in 3GPP LTE.

In a TDD frame, the PSS is transmitted in a third OFDM symbol of a thirdslot and a 13^(th) slot. The SSS is transmitted in the OFDM symbol whichis 3 OFDM symbols ahead of the OFDM symbol in which the PSS istransmitted. The PBCH is first 4 OFDM symbols of a second slot in afirst subframe.

FIG. 13 illustrates an example of a cell detection and a cell selectionthrough a synchronization signal.

Referring to FIG. 13( a), it is shown that a plurality of BSs, forexample, a first BS 200 a and a second BS 200 b are existed neighboringeach other, and a UE 100 is existed in an overlapped regiontherebetween.

First, each BS 200 a and 200 b transmits the PSS and the SSS asdescribed above.

Subsequently, the UE may receive the PSS from each BS 200 a and 200 b,and acquire cell IDs for the cells configured by each BS.

Next, each BS 200 a and 200 b also transmits a cell-specific referencesignal (CRS).

Herein, as known with reference to upper part of FIG. 13( b), as anexample, the CRS may be transmitted on 0^(th), 4^(th), 7^(th) and11^(th) OFDM symbols of a subframe.

In order to help understanding, what is CRS will be briefly described asfollows.

In 3GPP LTE system, two sorts of downlink reference signal, the CRS (oralso referred to as a common reference signal (RS)) and a dedicated RS(DRS, or also referred to as a UE-specific RS) are defined in order tofacilitate unicast service.

The CRS is a reference signal that is shared by all UEs in a cell, andis used for acquiring the information of channel state and measuringhandover.

A UE measures a reference signal received power (RSRP) and a referencesignal received quality (RSRQ) by measuring the CRS, and notifies it toa BS. Also, the UE notifies feedback information such as channel qualityinformation (CQI), pecoding matrix indicator (PMI) and rank indicator(RI), and the BS performs downlink frequency domain scheduling by usingthe feedback information received from the UE.

In order to transmit reference signals to the UE, the BS allocatesresources by considering the amount of radio resource that will beallocated to reference signal, the exclusive location of a commonreference signal and a dedicated reference signal, the location of asynchronization channel (SCH) and a broadcast channel (BCH) and densityof a dedicated reference signal, etc.

In this time, if the more resources are allocated to the referencesignal, higher channel estimation performance is obtainable, but thedata transmission rate is relatively decreased. And if the less resourceis allocated to the reference signal, higher data transmission rate isobtainable, but the density of reference signal becomes lower and thechannel estimation performance may be deteriorated. Accordingly, it maybe an important element in the system performance to allocate resourceseffectively for the reference signal considering the channel estimationand the data transmission rate.

Meanwhile, in 3GPP LTE system, the CRS is used for both objects of thechannel information acquisition and the data decoding. Particularly, theCRS is transmitted in every subframe in wideband, and the CRS istransmitted for each antenna port of a BS. For example, if there are twotransmission antennas in a BS, the CRS is transmitted through antennaports 0 and 1, and if there are four transmission antennas, the CRS istransmitted through antenna ports 0 to 3 respectively.

Referring to FIG. 13( b) again, a UE 100 receives the CRS from each BS200 a and 200 b, measures the RSRP and the RSRQ, and selects the cellthat has better RSRP and RSRQ values.

As such, when a cell is selected, the UE 100 may receive the PBCH from aBS that configures the selected cell, and acquire system informationthrough the PBCH. The system information may include, for example, theMIB above described. Also, the UE 100 receive the PDSCH from the BS thatconfigures the selected cell, and acquire the SIB through the PDSCH.

Meanwhile, the UE 100 enters the RRC connection mode through theselected cell.

In summary, after the UE 100 selects a proper cell firstly, the UE 100establishes the RRC connection in the corresponding cell, and registersthe information of UE in a core network. Later, the UE 100 is shifted toRRC rest mode and remained. As such, the UE 100 that is shifted to RRCrest mode and remained (re)selects a cell as occasion demands, and looksup system information or paging information. As such, when the UE thatis remained in the RRC rest mode is required to establish the RRCconnection, the UE establishes the RRC connection with the RRC layer ofE-UTRAN through the RRC connection procedure again, and shifted to theRRC connection mode. Herein, there are several cases that the UE in theRRC rest mode are required to establish the RRC connection again, forexample, the case of requiring uplink data transmission on the reasonthat a user tries to call, otherwise the case of transmitting a responsemessage when receiving a paging message from the E-UTRAN.

Meanwhile, in the next generation mobile communication system,multimedia broadcast/multicast service (MBMS) is suggested forbroadcasting service.

FIG. 14 illustrates an example of multimedia broadcast/multicast service(MBMS).

As known with reference to FIG. 14, within a service region, MBMS singlefrequency network (MBSFN) is applied such that a plurality of eNodeBs200 transmit the same date at the same time and in the same form.

The MBMS is referred to provide streaming or background broadcastservice or multicast service for a plurality of UEs by using downlinkdedicated MBMS bearer service. In this time, the MBMS service may bedivided into a multi-cell service that provides the same service for aplurality of cells and a single cell service that provides service foronly one cell.

As such, a UE receives the plurality of cell services, the UE mayreceive the same transmission of the plurality of cell servicestransmitted from several cells with being combined in the MBMS singlefrequency network scheme.

Meanwhile, by signaling the subframe in which the MBMS is transmitted tothe MBSFN subframe, the UE may know that.

FIG. 15 illustrates a hetero-network that includes a macro cell and asmall-scale cell.

In the communication standard of the next generation such as 3GPP LTE-A,there is a discussion about a hetero-network in which small-scale cellsthat have a low transmission power in the existing macro cell coverage,such as a pico cell, a femto cell or a micro cell is existed with beingoverlapped.

Referring to FIG. 15, a macro cell may be overlapped with one or moremicro cell. The service of macro cell is provided by a macro eNodeB(MeNB). In the present specification, the macro cell and the MeNB may beused with being mixed. A UE in connection with the macro cell may bereferred to as a macro UE. The macro UE receives downlink signals fromthe MeNB and transmits uplink signals to the MeNB.

The small-scale cell is also referred to as a femto cell, a pico cell ora micro cell. The service of small-scale cell is provided by a picoeNodeB, a home eNodeB (HeNB), a relay node (RN), etc. For theconvenience sake, the pico eNodeB, the home eNodeB (HeNB) and the relaynode (RN) are collectively referred to as a HeNB. In this specification,the micro cell and the HeNB may be used with being mixed.

The small-scale cell may be divided into an open access (OA) cell and aclosed subscriber group (CSG) cell according to accessibility. The OAcell signifies a cell in which a UE receives services anytime in case ofneed without separate access restriction. On the other hand, the CSGcell signifies a cell in which only a specific approved UE may receiveservices.

Since the macro cell and the small-scale cell are overlapped in thehetero-network, an inter-cell interference is a problem. As depicted, incase that a UE is located at a boundary between the macro cell and thesmall-scale cell, the downlink signal from the macro cell may act asinterferences. Similarly, the downlink signal of the small-scale cellmay also act as interferences.

As a detailed example, when the UE 100 that accesses the small-scalecell 300 is located at a boundary of the small-scale cell, theconnection between the UE and the small-scale cell may be disconnecteddue to the interference from the macro cell 200. This signifies that thecoverage of small-scale cell 300 becomes smaller than anticipated.

As another example, when the UE 100 that accesses the macro cell 200 islocated in an area of the small-scale cell 300, the connection with themacro cell 200 may be disconnected due to the interference from thesmall-scale cell 300. This signifies that a radio shadow area occurs inthe macro cell 200.

The most fundamental ways to solve the interference problem is to usedifferent frequency between the hetero-networks. However, since afrequency is rare and expensive resource, the way of solution throughfrequency division is not welcomed by the service provider.

Accordingly, in 3GPP, it has been tried to solve the problem ofinter-cell interference through the time division scheme.

According to this, in recent 3GPP, enhanced inter-cell interferencecoordination (eICIC) has been actively researched as a method ofinterference cooperation.

The time division scheme introduced in LTE Release-10 is called theenhanced inter-cell interference coordination (enhanced ICIC) as ameaning that it is an evolution in comparison with the existingfrequency division scheme. In the scheme, it is defined that each cellthat causes interference is referred to as an aggressor cell or aprimary cell, and the cell that receives interference is referred to asa victim cell and a secondary cell. The aggressor cell or the primarycell stops data transmission in a specific subframe, thereby enabling aUE to maintain access with the victim cell or the secondary cell in thecorresponding subframe. That is, in case that hetero-cells coexist, inthis scheme, a cell stops transmission of signal for a while for a UEthat receives significantly serious interference in a region, therebynot transmitting interference signal.

Meanwhile, the specific subframe in which the data transmission isstopped is called almost blank subframe (ABS), and in the subframe thatcorresponds to the ABS, any data is not transmitted except indispensiblecontrol information. The indispensible control information is, forexample, a cell-specific reference signal (CRS). In current 3GPPLTE/LTE-A standard, the CRS is existed in 0^(th), 4th 7^(th) and 11^(th)OFDM symbols in each subframe on time axis.

FIG. 16 illustrates an example of the enhanced inter-cell interferencecoordination (eICIC) to solve the problem of interference between BSs.

Referring to FIG. 16( a), if the small-scale cell 300 is a pico cell, amacro cell, i.e., the eNodeB 200 and the small-scale cell 300 thatcorresponds to the pico cell exchange the MBSFN subframe informationthrough X2 interface.

For example, the macro cell, i.e., the eNodeB 200 includes theinformation of MBSFN subframe and the information of subframe thatoperates as the ABS in MBSFN subframe Info information element (IE), andtransmits it to the small-scale cell 300 that corresponds to the picocell through a request message based on the X2 interface.

Meanwhile, the small-scale cell 300 that corresponds to the pico cellalso includes the information of MBSFN subframe and the information ofsubframe that operates as the ABS in MBSFN subframe Info informationelement (IE), and transmits it through a request message based on the X2interface.

In the meantime, as such, the macro cell, i.e., the eNodeB 200 and thesmall-scale cell 300 that corresponds to the pico cell may exchangeMBSFN subframe information through the X2 interface.

However, if the small-scale cell 300 is a femto cell, the small-scalecell 300 that corresponds to the femto cell does not have X2 interfacewith the macro cell, i.e., the eNodeB 200. In this case, in order forthe small-scale cell 300 that corresponds to the femto cell to acquirethe information of MBSFN subframe of the macro cell, i.e., the eNodeB200, the small-scale cell 300 that corresponds to the femto cell mayacquire the MBSFN subframe information by acquiring the systeminformation which is wirelessly broadcasted from the macro cell, i.e.,the eNodeB 200. Or, the small-scale cell 300 that corresponds to thefemto cell may also acquire the MBSFN subframe information of the macrocell, i.e., the eNodeB 200 from a control station of a core network.

Or, if the information of MBSFN subframe of the macro cell, i.e., theeNodeB 200 is fixed, the information of MBSFN subframe is applied to thesmall-scale cell 300 that corresponds to the femto cell throughoperations and management (OAM).

Referring to FIG. 16( b), a subframe is shown which the small-scale cell300 that corresponds to the pico cell configures as the MBSFN. When thesmall-scale cell 300 that corresponds to the pico cell configures thecorresponding subframe as the MBSFN and notifies it to the macro cell,i.e., the eNodeB 200, the macro cell 200 operates the correspondingsubframe as the ABS.

In the data region of the corresponding subframe, the small-scale cell300 that corresponds to the pico cell performs the data transmission,and the CRS is transmitted on the 0^(th), 4^(th), 7^(th), and 11^(th)symbols.

On the other hand, if the eICIC is applied, the macro cell, i.e., theeNodeB 200 does not transmit any data in the data region of thecorresponding subframe, and it prevents interference. However, the macrocell, i.e., the eNodeB 200 transmits only the corresponding subframeCRS.

By using the CRS received from the macro cell, i.e., the eNodeB 200 andthe small-scale cell 300 that corresponds to the pico cell respectively,the UE measures the reference signal received power (RSRP) and thereference signal received quality (RSRQ). For detailed example, if theserving cell of the UE 100 corresponds to the macro cell and thesmall-scale cell 300 that corresponds to the pico cell corresponds to aneighbor cell, the UE measures the RSRP and the RSRQ of the serving cellthrough the CRS of the macro cell 200, and measures the RSRP and theRSRQ of the neighbor cell through the CRS of the small-scale cell 300.

In the current 3GPP LTE/LTE-A standard, the cell-specific referencesignal (CRS) is existed in the 0^(th), 4^(th), 7^(th), and 11^(th) OFDMsymbols in each subframe on time axis. In the eICIC of LTE-A, for thecompatibility with the LTE UE, separate subframe is not used, but thealmost blank subframe (ABS) that does not allocate the data of theremaining part except the minimum signal required for the operation ofUE including the CRS is used. Also, in case of the MBSFN ABS subframe,by additionally eliminating the remaining CRS except the first CRS, theinterference among the CRSs is removed in the 4^(th), 7^(th), and11^(th) OFDM symbols that includes the remaining CRS except the firstCRS.

FIG. 17 illustrates a concept of expanding coverage of a small-scalecell.

As depicted in FIG. 17, within the coverage of a BS (i.e., an eNodeB)200 of a macro cell, a BS (i.e., a pico eNodeB) 300 of severalsmall-scale cells may be installed. And if a UE that has been receivedservice from the eNodeB 200 of the macro cell is existed in the coverageof the eNodeB 300 of the small-scale cell, the UE may handover to theeNodeB 300 of the small-scale cell, thereby obtaining the effect ofoffloading traffic of the eNodeB 200 of the macro cell.

Herein, the handover from the eNodeB 200 of the macro cell thatcorresponds to a serving BS to the eNodeB 300 of the small-scale cellthat corresponds to a target BS is performed when the strength ofreference signal of the target BS exceeds a specific threshold valuebased on the strength (RSRP, RSRQ) of the reference signal that the UE100 received from the serving BS.

However, by putting into a certain means additionally or by improvingcapability of the UE 100, it can be implemented that the handover intothe target BS may be performed even in case that the received referencesignal strength of the target BS does not exceed the threshold value ofthe received reference signal strength of the serving BS, andconsequently, such an operation gives birth to an effect of expandingthe cell boundary or the cell radius of the BS (i.e., the pico eNodeB)300 of the small-scale cell that corresponds to the target BS. In thedrawing, the expanded coverage area which is wider than the basiccoverage of the small-scale cell 300 is represented by deviant creaselines. Such an expanded coverage area may be referred to a cell rangeexpansion (CRE).

Herein, when representing the threshold value used for normal handoveras S_(th) _(—) _(conv), the area in which the CRE is available may berepresented as an area satisfying the condition, S_(th) _(—)_(covn)<=S_(received)<=S_(th) _(—) _(CRE).

Meanwhile, the reception strength for the reference signal from the BSof the small-scale cell 300 may be represented as the RSRP/RSRQ measuredin the UE 100. However, the RSRP/RSRQ may be measured only after the UE100 detecting, i.e., distinguishing the small-scale cell 300.

It will be described with reference to FIG. 18 in detail.

FIG. 18 illustrates the interference between signals of a macro cell andsynchronization signals of a small-scale cell and the interferencebetween reference signals.

As known from referring to FIG. 18( a), synchronization signals (i.e.,PSS and SSS) of a macro cell and synchronization signals (i.e., PSS andSSS) of a small-scale cell act as interference mutually. Accordingly, inorder for a UE 100 to properly receive the synchronization signal (i.e.,PSS and SSS) of the small-scale cell, the strength of noise-interferencesignal in comparison with received signal should be at least lower than6 dB.

However, in order to more increase the effect of offloading traffic intothe small-scale cell 300, if trying to forcibly handover the UE 100 inthe CRE area to the small-scale cell 300, firstly, the UE 100 in theexpanded coverage area, that is, the CRE area should be able to detectthe synchronization signal (PSS and SSS) of the small-scale cell.

In order to do that, the UE 100 should persistently use an interferenceremoving unit 119 for the synchronization signal (PSS and SSS) as shownin FIG. 10. Similarly, the UE 100 should persistently use theinterference removing unit shown in FIG. 10 also for the PBCH. Herein,the interference removing unit 119 of the UE 100 may include a PSS/SSSinterference removing unit, a PBCH interference removing unit, and a CRSinterference removing unit.

Particularly, since the UE 100 does not know whether the UE itself is inthe expanded coverage area or the CRE area, as far as the UE is providedwith the corresponding information from a serving BS, the UE shouldoperate the interference removing unit 119 always for the PSS/SSS andthe PBCH, and according to this, a power consumption is increased. Thisis very disadvantageous in an aspect of the battery capacity of UE.

In addition, referring to FIG. 18( b), the CRS of macro cell and the CRSof small-scale cell act as interference mutually. Accordingly, in orderfor the UE 100 in the expanded coverage area, i.e., the CRE area toproperly receive the CRS of small-scale cell, the interference removingunit should be always operated, and according to this, the powerconsumption becomes increased. This is very disadvantageous in an aspectof the battery capacity of UE.

However, if the UE 100 detects that it is located in the expandedcoverage area or the CRE area, and operate the PSS/SSS interferenceremoving unit, the PBCH interference removing unit, and the CRSinterference removing unit in the interference removing unit 119 only incase that the UE is located in the area (that is, the UE is located inan area in which an operation of the interference removing unit isrequired), the power consumption may be significantly decreased.

Accordingly, hereinafter, a method of determining when the interferenceremoving unit is operated in order for the UE 100 to perform a celldetection and measurement for the small-scale cell 300 will bedescribed. The method may be divided into a method by a UE and a methodby a BS. First, the method by a UE will be described with reference toFIG. 19 below.

FIG. 19 is a flowchart illustrating operation of UE according to anembodiment suggested in the present specification.

The UE 10 is able to measure the RSRP/RSRQ for the CRS received from aBS (i.e., the eNodeB 200 of a macro cell in the expanded coverage areaor the CRE area any time. Accordingly, the UE 100 may obtain Es/Iot ofthe CRS by using the RSRQ information as follows (step, S101).

$\begin{matrix}{{{CRS\_ Es}\text{/}{{Iot}\left\lbrack {dB} \right\}}} = {10\; \log \; 10{\left( \frac{10^{\frac{{R\; S\; R\; Q} + {10\log \; 10{(12)}}}{10}}}{1 - 10^{\frac{{R\; S\; R\; Q} + {10\; \log \; 10{(12)}}}{10}}} \right).}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Herein, Iot signifies total noise and received power spectrum density ofinterference for a specific resource element (RE) measured in an antennaconnector of UE. And, Es represents an energy received for RE during thevalid part (i.e., except cyclic prefix) of symbols.

Accordingly, the UE 100 compares the obtained value with two thresholdvalues (step, S103). That is, the UE determines whether the condition, afirst threshold value th1<=CRS_Es/Iot<=a second threshold value th2 issatisfied. The first threshold value is a lower limit value and thesecond threshold value is an upper limit value.

If the condition, the first threshold value th1<=CRS_Es/Iot<=the secondthreshold value th2 is satisfied, the UE 100 operates at least one ofthe PSS/SSS interference removing unit and the PBCH interferenceremoving unit (step, S105).

However, if the condition is not satisfied, the UE 100 does not operatethe PSS/SSS interference removing unit and the PBCH interferenceremoving unit.

The UE 100 itself may determine and configure the first threshold valueth1 and the second threshold value th2, but the first threshold valueth1 and the second threshold value th2 may be configured by a signal ofa BS of the macro cell.

FIG. 20 is a flowchart illustrating operation of a BS of a macro cellaccording to an embodiment suggested in the present specification.

First, an operation of a BS of a macro cell may be divided into twocases. First is the case that the BS of the macro cell transmits asignal, regardless of operating the PSS/SSS interference removing unitand the PBCH interference removing unit of a UE 100, such that the UE100 is available to handover from the expanded coverage area or the CREarea to the BS of a small-scale cell. Second is the case that even ifthe BS of the macro cell transmits a signal, the UE 100 is unable tohandover from the expanded coverage area or the CRE area to the BS ofthe small-scale cell, and it is required to operate the PSS/SSSinterference removing unit and the PBCH interference removing unit.

In the first case, it is important when the BS of the macro cellprovides a signal to the UE 100. In order to make this concrete, asshown in FIG. 20( a), when the eNodeB 200 of the macro cell receive ameasurement result (including RSRQ) which is reported from the UE 100(step, S211), by using the received measurement result (e.g., RSRQ), CRSEs/Iot is obtained through Equation 5 (step, S213). And the BS of themacro cell compares the obtained value with two threshold values (step,S215). For example, if the condition, a first threshold valueth1<=CR_SEs/Iot<=a second threshold value th2 is satisfied is satisfied,the BS of the macro cell determines that the UE 100 is located in theexpanded coverage area or the CRE area.

In addition, it may be implemented that the BS of the macro cell enablesthat the UE 100 may handover from the expanded coverage area or the CREarea to the BS of the small-scale cell by transmitting the signal (step,S217).

Meanwhile, the case may be occurred that it is miscalculated that the UE100 is in the location even if the UE 100 is not actually located in alocation where not the expanded coverage area (or the CRS area) of thesmall-scale cell. In order to prevent this, it may be implemented thatthe determination may be reversed or maintained by receiving the reportof the RSRP for the small-scale cell. In particular, in case that the UE100 is located in the expanded coverage area (or the CRE area), the RSRPfor the small-scale cell is normally reported. However, the RSRP for thesmall-scale cell reported in the miscalculated CRE area is not reportedsince the RSRP value is not existed. Accordingly, in case of maintainingthe determination, the eNodeB 200 of the macro cell may transmit aconfirm signal to the UE 100, and in case of reversing thedetermination, the eNodeB 200 of the macro cell may transmit acancellation signal to the UE 100.

In the second case, as shown in FIG. 20( b), when the eNodeB 200 of themacro cell receives a measurement result from the UE 100 (step, S221),the eNodeB 200 calculates the CRS Es/Iot value (step, S222) using theRSRQ information in the measurement result. And the eNodeB 200 comparesthis value with two threshold values (step, S225), then transmits thesignal for on/off command of the interference removing unit to the UE100 (step, S227). Particularly, if the condition, the first thresholdvalue th1<=CRS_Es/Iot<=the second threshold value th2 is satisfied, theeNodeB of the macro cell transmits a command of operating the PSS/SSSinterference removing unit and the PBCH interference removing unit.

Meanwhile, it may be implemented that the eNodeB of the macro celltransmits the two threshold values to the UE 100 instead of theoperating command such that the UE 100 performs the operation accordingto FIG. 19.

In the meantime, although it is described so far that the CRS Es/Iot isobtained using the RSRQ and the obtained value is used, in an alternateembodiment, the RSRQ may be used as it is. In this time, another twothreshold values thr1 and thr2 which are converted based on the RSRQ maybe used instead the two threshold values th1 and th2. In this case, thecondition may be changed as follows.

thr1<=RSRQ<=thr2

The embodiments of the present invention may be implemented by variousmeans. For example, the embodiments of the present invention may beimplemented in hardware, firmware, software or a combination thereof.

FIG. 21 is a block diagram illustrating a wireless communication systemwhere an embodiment of the present invention is implemented.

The base station 200 includes a processor 201, a memory 202, and an RF(radio frequency) unit 203. The memory 202 is connected with theprocessor 201 and stores various pieces of information for driving theprocessor 201. The RF unit 203 is connected with the processor 201 andtransmits and/or receives radio signals. The processor 201 implementsfunctions, processes, and/or methods as suggested herein. In theabove-described embodiments, the operation of the base station may beimplemented by the processor 201.

The wireless device 100 includes a processor 101, a memory 102, and anRF unit 103. The memory 102 is connected with the processor 101 andstores various pieces of information for driving the processor 101. TheRF unit 103 is connected with the processor 101 and transmits and/orreceives radio signals. The processor 101 implements functions,processes, and/or methods as suggested herein. In the above-describedembodiments, the operation of the wireless device may be implemented bythe processor 101.

The processor may include an ASIC (application-specific integratedcircuit), other chipsets, a logic circuit, and/or a data processingdevice. The memory may include an ROM (read-only memory), an RAM (randomaccess memory), a flash memory, a memory card, a storage medium, and/orother storage devices. The RF unit may include a baseband circuit forprocessing radio signals. When an embodiment is implemented in software,the above-described schemes may be realized in modules (processes, orfunctions) for performing the above-described functions. The modules maybe stored in the memory and executed by the processor. The memory may bepositioned in or outside the processor and may be connected with theprocessor via various well-known means.

In the above-described systems, the methods are described with theflowcharts having a series of steps or blocks, but the present inventionis not limited to the steps or order. Some steps may be performedsimultaneously or in a different order from other steps. It will beunderstood by one of ordinary skill that the steps in the flowcharts donot exclude each other, and other steps may be included in theflowcharts or some of the steps in the flowcharts may be deleted withoutaffecting the scope of the invention. The present invention may be usedin a user equipment, a base station, or other equipment of a wirelessmobile communication system.

INDUSTRIAL APPLICABILITY

The present invention may be used for a terminal, a base station orother equipment of wireless mobile communication systems.

What is claimed is:
 1. A method for detecting a small-scale cell in awireless communication system in which a macro cell and the small-scalecell coexist, the method performed by a user equipment which is locatedoutside a coverage of the small-scale cell and comprising: performing ameasurement for the macro cell; comparing a value related to a referencesignal received quality (RSRQ) obtained by the measurement for the macrocell with at least one threshold value; determining whether to operateat least one interference removing function in order to receive a signalfrom the small-scale cell, according to a result of comparing the value;and detecting the signal from the small-scale cell, if the interferenceremoving function is operated.
 2. The method of claim 1, wherein thecomparison step includes determining whether the value related to theRSRQ is between a lower limit value and an upper limit value.
 3. Themethod of claim 1, wherein the value related to the RSRQ is obtained by$\begin{matrix}{{{CRS\_ Es}\text{/}{{Iot}\left\lbrack {dB} \right\}}} = {10\; \log \; 10{\left( \frac{10^{\frac{{R\; S\; R\; Q} + {10\log \; 10{(12)}}}{10}}}{1 - 10^{\frac{{R\; S\; R\; Q} + {10\; \log \; 10{(12)}}}{10}}} \right).}}} & \;\end{matrix}$
 4. The method of claim 1, wherein the determining stepincludes determining whether to operate at least one of an interferenceremoving function for a primary synchronization signal (PSS) and asecondary synchronization signal (SSS), and interference removingfunction for a physical broadcast channel (PBCH).
 5. The method of claim1, further comprising: performing a cell reselection or a handover, if asignal is detected from the small-scale cell.
 6. A method forcontrolling a user equipment in a wireless communication system in whichthe macro cell and a small-scale cell coexist, the method performed by amacro cell and comprising: receiving, by the macro cell, a result of areference signal received quality (RSRQ) measurement for the macro cellitself from a user equipment which is located outside a coverage of thesmall-scale cell; comparing a value related to the RSRQ with at leastone threshold value; and transmitting a control signal for allowing theuser equipment to perform a cell reselection or a handover to thesmall-scale cell according to a result of comparing the value.
 7. Themethod of claim 6, wherein the control signal for allowing the userequipment to perform a cell reselection or a handover to the small-scalecell is either one of a command instructing the cell reselection or thehandover to the small-scale cell or a command for instructing the userequipment to operate an interference removing function.
 8. The method ofclaim 7, wherein the command for instructing the user equipment tooperate the interference removing function is a command for instructingthe user equipment to operate at least one of an interference removingfunction for a primary synchronization signal (PSS) and a secondarysynchronization signal (SSS), and an interference removing function fora physical broadcast channel (PBCH).
 9. The method of claim 6, whereinthe comparison step of the value includes determining whether the valuerelated to the RSRQ is between a lower limit value and an upper limitvalue.
 10. The method of claim 6, wherein the value related to the RSRQis obtained by $\begin{matrix}{{{CRS\_ Es}\text{/}{{Iot}\left\lbrack {dB} \right\}}} = {10\; \log \; 10{\left( \frac{10^{\frac{{R\; S\; R\; Q} + {10\log \; 10{(12)}}}{10}}}{1 - 10^{\frac{{R\; S\; R\; Q} + {10\; \log \; 10{(12)}}}{10}}} \right).}}} & \;\end{matrix}$